multiplayerchess.com
Reasonably easy to find a specific opponent (Just click the "Play With Your Friends" and once a game is made you'll receive an URL to send to the person you'll be playing against."
It's not the best site out there, but it's probably the fastest I know in regards to 'finding an specific opponent' (since you just need to send the URL, really.)
The site itself isn't all that, though.
No idea for rating system.